Dark matter took its time to wrap around early galaxies

Darkness gathers – but it can take time. About 10 billion years ago, massive star-forming galaxies were dominated by normal matter, not the dark matter that’s so influential in galaxies today.

If spiral galaxies were only made up of the matter that we can see, stars at the outer edge should orbit the centre slower than those closer in. But in the 1970s, American astronomers Vera Rubin and Kent Ford noticed that this was not the case: all the stars in the Andromeda galaxy move at similar speeds, regardless of their distance from the galactic center.

This study constituted some of the first evidence for dark matter, matter that doesn’t interact with light and which we can only observe via its gravitational effects. If a galactic disc sat in a pervasive dark matter “halo”, that extra unseen mass could explain the stars’ unexpected motion.

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In order to figure out how today’s galaxies came to be so full of dark matter, we have to look to their predecessors, the star-forming galaxies of the early universe. But learning about dark matter in galaxies in the early universe is difficult: because we cannot see dark matter, we must carefully observe the movements of extraordinarily distant stars.

Now, Natascha Förster Schreiber at the Max Planck Institute for Extraterrestrial Physics in Germany and her colleagues have used the Very Large Telescope in Chile to make the most detailed observations yet of the movement of six massive galactic discs during the peak era of galaxy formation, 10 billion years ago.

Slow spin

They found that, unlike in most modern galaxies, the stars at the edges of these early galaxies move more slowly than those closer in. “This tells us that at early stages of galaxy formation, the relative distribution of the normal matter and the dark matter was significantly different from what it is today,” says Förster Schreiber.

In order to check their unexpected results, the researchers used a “stack” of 100 images of other early galaxies to find an average picture of their rotations. The stacked galaxies matched the rotations of the more rigorously studied ones. “We’re not just looking at six weirdo galaxies – this could be more common,” says Förster Schreiber. “For me, that was the wow moment.”

The differences in early galaxies’ rotations demonstrates that there is very little dark matter in towards their middle. Instead, they are almost entirely made up of the matter we can see in the form of stars and gas. The further away (and thus earlier in cosmic history) the galaxies were, the less dark matter they contained.

This result suggests that the turbulent gas in early galaxies condensed into the flat, rotating disc shapes we see today more quickly than dark matter, which remained in a diffuse halo around galaxies for longer.

“This is an important step in trying to figure out how galaxies like the Milky Way and larger galaxies must have assembled,” says Mark Swinbank at Durham University. “Having a constraint on how early the gas and stars must have formed the discs and how well-mixed they were with dark matter is important to informing their evolution.”